The present invention relates to electron beam guns for an electron beam evaporation source of the type used in depositing a thin film of evaporated material. More particularly, the present invention relates to several improvements in electron beam guns.
Electron beam guns for an electron beam evaporation source are well known in the art. Exemplative of electron beam guns are those disclosed in U.S. Pat. No. 5,182,488 which is assigned to The BOC Group, Inc. and is hereby incorporated by reference.
Referring to FIG. 1, an electron beam gun of the prior art is shown. Electron beam gun 10 is provided with a cathode block 12 having two conductive sides 14 and 16 spaced slightly apart from one another. A filament 18 is connected to sides 14 and 16 of filament block 12 by two filament clamps 20 and 22, affixed to the two sides 14 and 16 of cathode block 12. Two filament buss bars 28 and 30 are in turn connected to sides 14 and 16 of filament block 12 to apply an electric current across sides 14 and 16 of cathode block 12, and thus, filament 18. The electric current causes filament 18 to emit electrons.
Filament buss bars 28 and 30 are in turn connected to mounting bar 40. Cathode block 16 is insulated from mounting bar 40 by provision of 2 sets of insulative washer pairs 42 and 44. One of the filament blocks must be insulated from mounting bar 40 to prevent sides 14 and 16 of cathode block 12 from being shunted. Mounting bar 40 is in turn connected at its ends to a support bar 46 by screws which pass through insulator pairs 52 and 54. Insulator 54 slides into shield 52 in a manner well known in the art.
An anode 56 is provided to accelerate electrons emitted by filament 18. Anode 56 is connected at the bottom to anode mounting bracket 58. Anode mounting bracket 58, at its ends, is in turn attached to support bar 46. Anode 56 has a top portion 64 of inverted U-shaped configuration which terminates in a front edge 66 in front of filament 18.
A beam former 68 is provided. Beam former 68 is configured to shield filament 18 from anode 56. Beam former 68 is located between filament 18 and anode 56, and has essentially the same shape as anode 56. That is, it has a top portion 70 of inverted U-shaped configuration terminating in a lower edge 72 located between lower edge 66 of anode 56 and filament 18.
The shielding provided by beam former 68 causes electrons emitted by filament 18 to form an electron cloud under top portion 70 of beam former 68. As a result, a virtual cathode is formed from which electrons pass beneath lower edge 72 of beam former 68 in a flat ribbon-like configuration toward lower edge 66 of anode 56. By the time electrons reach lower edge 66 of anode 56, a sufficient kinetic energy has been imparted such that the ribbon-like beam misses anode 56 and is emitted from electron beam gun 10.
In earlier prior art, beam former was formed in a single section and was connected to one side of cathode block. The other side of beam former was insulated from the other side of cathode block to prevent cathode block from being shunted. The end result of this was that a thermal gradient was produced within beam former in which beam former was coolest at its attachment side of cathode block 12.
In such earlier prior art devices, the thermal gradient within the beam former caused warpage and translational and rotational movement which in turn caused electrons emitted by the filament to directly arc and strike the anode as well as cause deformation of the ribbon-shaped beam to produce non-uniform evaporation as the electron beam gun aged over its service life.
In order to overcome the disadvantages of warping and translational and rotational movement resulting from the thermal gradient within the beam former, the prior art beam former 68 is formed of two sections 74 and 76 separated by a gap 78. Sections 74 and 76 of beam former 68 are separately connected to sides 14 and 16 of cathode block 12.
The beam former deflects the electrons produced by the filament so that they pass by the edge of the anode instead of actually hitting the anode. Misalignment of the beam former or the anode can result in damage in the form of erosion or melting of the anode.
One group of prior art electron beam guns uses a beam former that is attached to and cantilevered off of one of the support members or cathode blocks. In this configuration, the beam former is spaced off of and insulated from the adjacent cathode block, usually by a ceramic spacer. One of the problems with these prior art arrangements is that the beam former can become misaligned by loosening, bending or warping during use and thermal cycling. Since the beam former is only secured at one side and extends across the hot filament in this cantilever arrangement, misalignment of the beam former is likely.
In another group of prior art electron beam guns and in particular, the electron beam gun discussed above, the beam former is separated into two sections with each section attached to its own support member. Such configuration of the beam formers is somewhat more stable since they are shorter and not cantilevered; however the potential for misalignment is significant. If the gap between the two halves of the beam former becomes too large, a significant amount of electrons can pass through this gap resulting in erosion and melting of the beam former edges as well as melting of the anode. Moreover, a short circuit could occur if the two halves of the beam former come into contact with each other due to shifting of the cathode blocks or beam former during use and thermal cycling. In both of these prior art designs, it takes significant effort to place the critical edge of the beam former in the correct location since it is a separate part or parts from the cathode blocks.
In addition, since the anode is a thin structure with supports located away from and on a separate face other than the critical alignment face directly in front of the beam former, anode warping can progress to the point of disalignment which degrades the performance of the electron gun.